An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun, such as that shown in U.S. Pat. No. 3,772,554, issued to R. H. Hughes on Nov. 13, 1973, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having the bottoms of the members facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In such electron guns, static convergence of the outer beams with respect to the center beam is usually attained by offsetting the outer apertures in the second focusing electrode with respect to the outer apertures in the first focusing electrode.
It has been noted that the horizontal beam landing locations of the outer electron beams, in color picture tubes having the above-described electron gun, change with changes in the focus voltage applied to the electron gun. It therefore is desirable to improve such inline electron guns to eliminate or at least reduce this sensitivity to focus voltage changes.
U.S. Pat. No. 4,449,069 issued to N. Z. Assil et al., on May 15, 1984 and assigned to the same assignee as the present invention, proposes a structure for reducing the horizontal motion of the outer electron beams caused by changes in the focus voltage. In the Assil et al. electron gun structure, designed to operate at an ultor voltage of 25 kV and a focus voltage of 7 kV, two slot apertures are formed in a portion of a focusing electrode facing a screen grid electrode. The slot apertures are closedly spaced to and outward from the two outer beam apertures of the focusing electrode. The slot apertures cause a distortion of the electrostatic field formed between the focusing electrode and the screen grid electrode at the outer beam apertures to converge the two outer beams toward the center beam. The spacing between the additional slot apertures and the outer beam apertures in the Assil et al. structure, i.e. the web spacing, is in the range of 0.60 mm to 1.50 mm. Since the thickness of the focusing electrode is only 0.25 mm, the metal web separating the beam apertures from the slot apertures does not possess a great deal of structural strength and may become distorted during the beading operation when alignment pins are positioned within the beam apertures. Since the electron beams have a high velocity in the vicinity of the focusing electrode, it is not feasible to move the slot apertures in the focusing electrode further outward from the outer beam apertures to strengthen the web, because the effect of the slot apertures on the electrostatic field would be decreased to the point where little reconvergence of the outer beams toward the center beam would occur. In some applications, it is desirable to operate the electron gun at 30 kV ultor potential with a focus voltage of 8.5 kV. In such an electron gun, the electron beams in the vicinity of the focus electrode would experience an even higher velocity than described above for an electron gun operation at an ultor potential of 25 kV and a focus voltage of 7 kV. It is known that the electron velocity is proportional to the square root of the voltage; therefore, to reconverge the outer beams using the teaching of the Assil et al. patent would require locating the slot apertures closer than the above-described range of 0.60 mm to 1.50 mm. Such a positioning would reduce the web spacing between the outer beam apertures and the slot apertures to the point where distortion of the outer beam apertures of the focusing electrode would almost certainly occur.